1. Field of the Invention
The present invention generally relates to projection systems and, more particularly, to reducing visual artifacts within such systems.
2. Background of the Invention
Digital Light Processing™ technology refers to the use of an optical integrated circuit that manipulates a light source. The optical semiconductor, referred to as a digital micromirror device (DMD), can be incorporated into a larger projection system. Products incorporating DMD technology are manufactured for a variety of different applications including home and commercial theater use.
A DMD can include a rectangular array of approximately 1.3 million microscopic mirrors called micromirrors. Each micromirror is extremely small, measuring less than approximately one-fifth the width of a human hair. Each micromirror further is mounted on hinges that enable the micromirror to tilt either toward a light source or away from the light source under the control of a driver circuit. When tilted toward the light source, the mirror is said to be in an “on” position since light from the light source is reflected. When tilted away from the light source, the mirror is said to be in an “off” position as no light from the light source is reflected.
A control signal provided to the DMD from the driver circuit directs each micromirror to switch on and off up to several thousand times per second. When a micromirror is switched on more frequently than off, the micromirror reflects a light gray pixel. Conversely, a micromirror that is switched off more frequently than on reflects a darker gray pixel. Thus, when a light source emitting white light is combined with a DMD and a lens, a gray-scale projection system is formed.
Color projection systems can be built by inserting a color wheel in the light path the between the DMD and the light source, or in the light path between the DMD and the projection lens. At the edges of the color wheel are light filters for generating red, green, and blue light. White light emanating from the light source can be passed through these filters as the color wheel rotates. The resulting colored light ultimately enters a projection lens for display on a screen.
The rotation of the color wheel is coordinated with control signals provided to the DMD. Thus, each micromirror can be switched on and off at a particular rate, or for selected time periods, that can vary according to which light filter of the color wheel is being used to filter the light source for a given time period. In other words, the on and off states of each micromirror are coordinated with the rotation of the three colored light filters of the color wheel.
In illustration, a micromirror intended to generate a purple pixel can be turned on to only to cause the reflection of red and blue light. That is, the micromirror can be switched on more often than off when the red and blue filters are used to filter the light. The red and blue reflected light is perceived as a particular shade of purple when shown in rapid succession in the same projection space. In this manner, each micromirror can project what is perceived to be a color pixel of an image. As noted, the switching of the micromirrors, and the proportion of time each micromirror is on or off, is coordinated according to the colored light filter being used to filter the light source. The human visual system integrates the sequential color images and sees a multicolored image.
The color wheel passes only a single color of light through corresponding to the particular one of the filters aligned with the light incident on the wheel. This leads to a situation where images of red, green, and blue are not time coincident upon the projection surface. Small time delays between consecutive colored images can cause noticeable visual artifacts in the resulting image. If the viewer's eye moves too rapidly, the viewer may perceive the individual red, green, and blue images. Such can be the case even when the object is supposed to be white. This can be perceived as a rainbow artifact, meaning that the different colored images are not being perceived as a single, blended image.
It would be beneficial to provide a projection system using DMD technology that reduces perceivable visual artifacts, and particularly rainbow artifacts.